In this alternate history, the POD is set during the Siege of Vienna by the Ottoman Turks in 1529 (Not to be confused with the one in 1683). In real life, the battle was a decisive Austrian victory which prevented Austria's complete annihilation by the Ottomans. However, in this alternate history, the Ottomans win a Pyrrhic victory at Vienna and continue their offensive against Austria.
The Siege of Vienna
After a long march toward the city, on the 27th of September the Ottomans besieged Vienna. While initially weather and strong defences by the Austrians hindered the Ottomans, by January of 1530, reinforcements arrived and the force besieging the city grew from 120,000 to 180,000. The Ottomans tactics mostly involved attempting to sabotage the cities wall defences, including mines and digging tunnels beneath the walls. Suleiman's plan was to continue this until they managed to break through. However in late February, he received news of a potential Austrian counter-attack that could crippled the besieging force.
Needing to capture the city, Suleiman ordered cannon-fire be centered toward the already-damaged points of Vienna. Heavy snow hindered his efforts, and at one point Suleiman was even forced to halt his attack in order to prevent his cannons and soldiers from freezing. However, on March 11th, using a mix of his original tactics and concentrated cannon-fire, the Ottoman's broke through the walls and stormed the city, killing the garrison and occupying Vienna.